1. Field of the Invention
Static pressure is an important parameter that is utilized in aircraft for numerous purposes including the determination of altitude, altitude rate, airspeed and airspeed rate, which last-mentioned parameters are, in turn, used as inputs to various other devices such as flight management systems, autopilots, ground proximity warning systems and wind shear detectors. Such systems are sensitive to errors in airspeed and airspeed rate, as well as altitude rate, caused by errors in static pressure readings. In particular, wind shear detection and warning systems are particularly sensitive to airspeed rate errors can give false warnings as a result of erroneous airspeed rate measurements thus reducing the pilot's confidence in the instrument.
2. Description of the Prior Art
Airspeed in an aircraft is determined by taking the difference in pressure between a ram pressure obtained from a pitot tube located in the airstream around the aircraft and a static pressure taken from a static port located in a region of relatively undisturbed air flow along the aircraft optimized to provide minimum altitude errors in steady state cruise. In some airspeed systems, the static port is collocated with the pitot port. These systems are not subjected to the same airspeed errors. In early airspeed indicators, the static pressure and the ram pressure were applied to opposite sides of a diaphragm that was deflected by the difference in the two air pressures. The diaphragm was mechanically coupled to a dial that was calibrated in airspeed. During high angle of attack or high angle of attack rate maneuvers, static pressure errors affect the airspeed readout. Correction tables are published in the Flight Manual by the aircraft manufacturer for aircraft where this error is significant. The pilot has no other absolute reference, so he does not know that the indicated airspeed is in error. In addition, some modern aircraft such as large transport aircraft encounter large variations in static pressure during extreme maneuvering, and such variations can cause significant errors in airspeed and airspeed rate measurements because, in such aircraft, the static port is generally spaced a long distance from the pitot tube and dynamic static pressure variations at the static port do not correspond to those of the pitot tube.
The static probe location is optimized to provide minimum altitude errors in cruise on some aircraft and the dynamic pressure variations at the static port are reduced by providing a lag can at the static port. Thus, any pressure variations entering the static port enter the lag can and are integrated by the volume of the lag can before being applied to any transducer. While such a lag reduces the dynamic variations in the static pressure during maneuvering, the integrating action of the lag can results in a one to two second delay in the static pressure measurement. While such a delay does not significantly impair the operation of standard instruments such as airspeed indicators and barometric altimeters, it is troublesome to certain newer instruments, such as wind shear detection and warning systems.
Dynamic altitude rate errors have been minimized in the past by using inertial vertical speed computed in a complementary filter which combines short term inertial vertical speed with long term barometric vertical speed. Similarly, airspeed and airspeed rate errors due to static pressure errors have been corrected by combining longitudinal inertial acceleration with airspeed in a complementary filter. For wind shear detection systems, this method cannot be used because it defeats the purpose of detecting wind shear, namely, the difference between inertial and airmass speed.
Wind shear detection and warning systems monitor airspeed or airspeed rate and compare it with an inertially-derived velocity or acceleration, and if the airspeed or airspeed rate diverges substantially from the corresponding inertially-derived velocity or acceleration, a wind shear warning is given. However, since airspeed and airspeed rate are derived from the difference between pitot pressure and static pressure, any erroneous dynamic variations in static pressure caused by maneuvering or other aircraft related factors can cause errors or variations in the airspeed or airspeed rate measurements of a sufficient magnitude to affect the operation of a wind shear warning system significantly and to cause the system to give false warnings, thereby eroding pilot confidence in the device.